1. Field of the Invention
This invention relates to a communication equipment that uses an OFDM (orthogonal frequency division multiplexing) system and, particularly, to a communication equipment which effectively suppresses peaks in the transmission.
2. Description of the Related Art
Signals are communicated by using the OFDM system in a mobile station equipment such as a cell phone in a mobile communication system, in a base station equipment that communicates with the mobile station equipment and in a communication station equipment in a digital television (TV) broadcast system.
FIG. 6 illustrates the constitution of an OFDM transmitter provided in a radio communication equipment, which is the constitution of a conventional multi-carrier synthesizing transmitter with a peak suppressing function (peak limiter). In this conventional example, the OFDM signals of the two carriers are synthesized.
The OFDM transmitter of this conventional example includes two OFDM modulators 101 and 102, two intermediate-frequency (IF) converter circuits 103 and 104, a peak-suppressing circuit 105 that receives two carrier OFDM signals and suppresses the peaks, a synthesizer 106, a radio frequency (RF) converter unit 107, a transmission amplifier (transmission AMP) 108 and an antenna 109.
Here, the constitution and operation of the OFDM transmitter of this conventional example are different from those shown, for example, in FIG. 1 with respect to that no BB peak suppressing circuit 3 is provided, but are roughly the same as those of FIG. 1 concerning other constitution and operation.
FIG. 7 illustrates the constitution of a conventional multi-carrier synthesizing peak-suppressing circuit 105.
The peak-suppressing circuit 105 of this conventional example includes a synthesized amplitude operation circuit 110, a peak suppress value operation circuit 111, two multiplier circuits 112 and 113, two delay elements 114 and 116, two filters 115 and 117, and two subtractor circuits 118 and 119. These circuits are all digital circuits.
Though this conventional example illustrates a constitution for transmitting two carriers, three or more carriers can be transmitted by adding the same circuits in a number corresponding to the number of carriers to be transmitted. When one carrier is to be transmitted, the number of circuits can be decreased down to, for example, one. In the constitution of FIG. 7, however, the circuit can be operated without any change by inputting no signal to the carrier side where there is no input.
Described below is the operation of the peak-suppressing circuit 105 of this conventional example.
An IF1 signal (A) and an IF2 signal (B) which are OFDM signals of one carrier having different center frequency in the IF are input through the IF converter circuits 103 and 104 of each of the series, and are separately compressed for their peaks based on their predetermined synthesized amplitudes. The two IF signals are, respectively, input as digital complex signals having an I (in-phase) component and a Q (quadrature-phase) component.
The synthesized amplitude operation circuit 110 finds a synthesized amplitude after having synthesized the vectors of the IF1 signal (A) and the IF2 signal (B) input through the IF converter circuits 103 and 104, and outputs the synthesized amplitude to the peak suppress value operation circuit 111. Here, the synthesized amplitude of this conventional example becomes an absolute value (Abs) obtained by synthesizing the IF1 signal (A) and the IF2 signal (B) together, and is expressed by the formula (1),Synthesized amplitude=Abs(A+B)  (1)
The peak suppress value operation circuit 111 compares a value of the synthesized amplitude obtained by the synthesized amplitude operation circuit 110 with a value set by a limiter, executes the operation of the formula (2) when the value of the synthesized amplitude exceeds a threshold value, and outputs the obtained value P to the multiplier circuits 112 and 113. When the value of the synthesized amplitude does not exceed the threshold value, the peak suppress value operation circuit 111 outputs the value 0 to the multiplier circuits 112 and 113.P=1−(threshold value/synthesized amplitude)  (2)
The multiplier circuits 112 and 113 of each of the series multiply the IF signals (IF1 signal (A) and IF2 signal (B)) from the IF converter circuits 103 and 104 of each of the series by the value (P or 0) from the peak suppress value operation circuit 111, and output the multiplied results to filters 115 and 117 of each of the series.
The filters 115 and 117 of each of the series have complex tap coefficients and, as frequency characteristics of the filters, have coefficients with the band of OFDM transmitted-signals of different intermediate frequencies as a pass band. The filters 115 and 117 of each of the series work to filter the signals input from the multiplier circuits 112 and 113 of each of the series, and output them to the subtractor circuits 118 and 119 of each of the series.
The delay elements 114 and 117 of each of the series correct the delay of processing by the filters 115 and 117 of each of the series, delay the IF signals (IF1 signal (A) and IF2 signal (B)) from the IF converter circuits 103 and 104 of each of the series, and output them to the subtractor circuits 118 and 119 of each of the series.
The subtractor circuits 118 and 119 of each of the series subtract impulse response signals from the initial IF signals (IF1 signal (A) and IF2 signal (B)) input from the delay elements 114 and 116 of each of the series, the impulse response signals being the ones obtained by multiplying, by a band limit of transmitted signals, the sample signals that are input from the filers 115 and 117 of each of the series and are to be suppressed for their peaks. The subtraction circuits 118 and 119 output the subtracted results to the synthesizer 106. The output signals to the synthesizer 106 are the ones of which the peaks are suppressed while holding the band of the transmitted signals.
In this example, the value P is multiplied by the sample of which the peak is to be suppressed for each of the carriers. Therefore, even if the carrier levels are not balanced causing a difference in the transmitted electric power between the carriers, the multiplier circuits 112 and 113 so work (as to evenly) distribute peak suppress signals to each of the carriers depending upon the transmission levels, making it possible to nearly uniform the EVM (error vector magnitude) which is a distortion scale for evaluating the quality of the transmitted signals between the carriers so as to be adapted to the carrier levels that are lacking balance.
In the OFDM transmitter of this conventional example as described above, the peak-suppressing signals are limited for their band making it possible to maintain frequency characteristics of the transmitted signals and to suppress the peak satisfying the standard of transmission spectrum.    Patent document 1: European patent No. 1469649
However, the OFDM signal has a large peak to average power ratio (PAPR) and requires a large peak-suppressing amount for improving the efficiency of the transmission amplifier. On the other hand, the modulation system in many cases uses a multi-value modulation system like 64 QAM (quadrature amplitude modulation) for transmitting data at high speeds and, therefore, the transmitted signals must satisfy the standards of transmission frequency characteristics yet maintaining the EVM low and maintaining a large peak-suppressing amount. In the conventional OFDM transmitter, however, if it is attempted to increase the peak-suppress amounting, a problem occurs in that the deterioration in the EVM is not avoidable. To realize equipment, further, a problem remains concerning how to decrease the scale of the circuit.